The present invention relates to the communication of signals via optical fibers, and particularly to an optical fiber carrying a number of channels multiplexed at different wavelengths. More particularly, the invention relates to gratings and their interaction with an optical waveguide such as an optical fiber.
Optical fibers are being used with increasing regularity for the transmission and processing of optical signals. Dense wavelength division multiplexing (DWDM) enables an individual optical fiber to transmit multiple channels simultaneously, the channels being distinguished by their center wavelengths. A need exists for a wavelength sensitive reflector that can be used as a component of an optical fiber system. Such devices are disclosed in U.S. Pat. No. 4,400,056 (xe2x80x9cEvanescent-Wave Fiber Reflectorxe2x80x9d, Aug. 23, 1983, Cielo) and U.S. Pat. No. 4,986,624 (xe2x80x9cOptical Fiber Evanescent Grating Reflectorxe2x80x9d, Jan. 22, 1991, Serin, et al). As disclosed in U.S. Pat. No. 4,400,056 (xe2x80x9cEvanescent-Wave Fiber Reflectorxe2x80x9d, Aug. 23, 1983, Cielo) a grating is developed on a photoresist deposited on the etched cladding of an optical fiber, whereas disclosed in the patent U.S. Pat. No. 4,986,624, (xe2x80x9cOptical Fiber Evanescent Grating Reflectorxe2x80x9d, Jan. 22, 1991, Serin, et al) a periodic grating structure is placed on a facing surface formed on an optical fiber. In both patents the grating structure is within a portion on the evanescent field of an optical signal propagating through the optical fiber. In both patents the spatial periodicity of the grating structure is selected to be equal to one-half the propagation wavelength of the optical signal. The grating structure causes the optical signal to be reflected at an angle 180 degrees and thus to propagate in a reverse direction from its original direction of propagation.
Other constructions of optical reflectors known as Bragg filters are gaining popularity. One type of Bragg filter is incorporated or embedded in the core of an optical fiber by a method disclosed, for instance in U.S. Pat. No. 4,807,950 (xe2x80x9cMethod for Impressing Gratings Within Fiber Opticsxe2x80x9d, Feb. 28, 1989, Glenn, et al). As is discussed in this patent permanent periodic gratings of this kind can be provided or impressed in the core of an optical fiber by exposing the core through the cladding to an interference pattern of two coherent beams of ultraviolet light that are directed against the optical fiber symmetrically to a plane normal to the fiber axis. This results in a situation where the material of the fiber core has permanent periodic variations in its refractive index impressed therein by the action of the interfering ultraviolet light beams. The periodic variations in refractive index oriented normal to the fiber axis, constitute the Bragg grating. An embedded Bragg grating of this kind reflects the light launched into the fiber core for guided propagation. Only that light having a wavelength within a very narrow range dependent on the grating element periodicity is reflected back along the fiber axis opposite to the original propagation direction. To light at wavelengths outside the aforementioned narrow band the grating will be substantially transparent, and will not adversely affect the further propagation of such light. In effect, this type of grating creates a narrow notch in the transmission spectrum, and by the same token a similarly narrow peak in the reflection spectrum. Further development is disclosed in U.S. Pat. No. 5,007,705 (xe2x80x9cVariable Optical Fiber Bragg Filter Arrangementxe2x80x9d, Apr. 16, 1991, Morey, et al) which relates to different aspects or uses of these discovered principles. In this patent various means are disclosed for intentionally shifting the reflection wavelength response of a Bragg grating. By deliberately varying the period of the grating or altering the index of refraction in a predetermined manner, by external forces or actions on the fiber section containing the grating in a controlled manner, a variable light filtering element is provided. Furthermore, tuning a grating by various means such as the application of heat, compression, or stretching are all known. Disadvantageously, these methods only allow the selection of a single wavelength to be reflected. Such mechanical solutions may be bulky, potentially unreliable, and have a slow response time.
U.S. Pat. No. 5,446,809 (xe2x80x9cAll Fiber Wavelength Selective Optical Switchxe2x80x9d, Aug. 29, 1995, Fritz, et al). discloses an optical wavelength selective optical switch, utilizing tunable Bragg fiber gratings. The fiber wavelength selective switch has one or more input optical couplers and utilizes a plurality of in-line Bragg fiber gratings in series along multiple parallel paths. For a given wavelength of light to pass through a particular grating, the grating must be detuned. By providing a plurality of Bragg gratings in series, each designed to reflected a different wavelength, and having means for controlling or shifting the response of each grating individually, signals can selectively be passed through a fiber or can be reflected backwards in a binary on-off fashion. The non-binary response version is disclosed in U.S. Pat. No. 5,699,468 (xe2x80x9cBragg Grating Variable Optical Attenuatorxe2x80x9d, Dec. 16, 1997, Farries, et al).
The invention according to a first broad aspect provides an arrangement having an optical waveguide, a grating, such as a Chirp grating, and an electro-optic material arranged adjacent the Chirp grating, the electro-optic material, for example liquid crystal material, having a refractive index which can be varied by applying an electric field to the material so as to control how the Chirp grating interacts with the optical waveguide. The Chirp grating has a spatial period that increases gradually along a length of the grating.
In the arrangement, the optical waveguide may have a region of cladding made of the electro-optic material, with the Chirp grating arranged adjacent the region of cladding to cause a reflection of particular wavelengths of light when propagating within the optical waveguide, the arrangement being adapted to receive the applied electric field which controls how the Chirp grating interacts with the waveguide.
In one embodiment, the arrangement is adapted to receive the applied electric field in at least one selectable region of the electro-optic material. The at least one selectable region may consist of an interaction region in which the Chirp grating will interact with optical signals in the optical waveguide, regions of the electro-optic material outside of the interaction region defining a blocked region in which the Chirp grating will not interact with optical signals in the optical waveguide, the selection of the interaction and blocked regions controlling which wavelengths of light are reflected.
Typically, the interaction region and blocked region are defined by applying the electric field to the electro-optic material so as to cause an increased index of refraction in the interaction region as compared to the index of refraction in the blocked region.
Wavelength selectivity may be achieved by selectively applying the electric field to appropriate portions of the electro-optic material thereby causing interaction with an appropriate portion of the spatial Chirp grating.
Preferably, electrodes are provided and arranged to cause reflection of any polarization of the wavelengths to be reflected, for example by applying first and second electric fields which are substantially perpendicular to each other.
For example, the arrangement might be adapted to apply the first electric field with a first plurality of electrodes of first polarity on a first side of the electro-optic material and a second plurality of electrodes of second polarity on a second side of the electro-optic material, and to apply the second electric field with a third plurality of electrodes of first polarity on a first side of the electro-optic material and a fourth plurality of electrodes of second polarity on a second side of the electro-optic material.
In any of the above embodiments, the waveguide might for example be a optical fiber with a portion of cladding removed from where the electro-optic material is located.
In one embodiment, the arrangement has a bottom substrate on which the Chirp grating is located, and a top substrate adapted to hold the optical fiber parallel to the Chirp grating. The electro-optic material is then sandwiched between the bottom substrate and the top substrate.
Any of the above arrangements might further include a housing assembly, an input connector and an output connector, and may be provided in combination with a voltage source and a controller adapted to control to which of the electrodes a voltage is applied to so as to apply the electric field.
Preferably, the Chirp grating has a spatial periodicity which increases along its length from xcex9A to xcex9B, where xcex9A=xcexA (2 neff) and xcex9B=xcexB/(2 neff), and xcexA to xcexB is an operational wavelength range of the arrangement. To reflect a wavelength xcexC, xcexAxe2x89xa6xcexCxe2x89xa6xcexB, an electric field is applied to a region of the Chirp grating having xcex9C=xcexC/(2 neff).
In another embodiment, a DWDM optical communication system which includes one or more of the above defined arrangements is provided.
The invention in accordance with another broad aspect provides an arrangement such as described previously further adapted to function as a wavelength selectable variable optical attenuator.